1) Field of the Invention
The present invention relates to an organic light emitting diode display device including organic light emitting diode elements as light emission elements, and each of the organic light emitting diode elements is covered with a protective layer. More specifically, the present invention relates to the organic light emitting diode display device having a stress relaxation layer for relaxing a stress caused by the protective layer, and a method of manufacturing the organic light emitting diode display device.
2) Description of the Related Art
Organic light emitting diode displays are now attracting attention as candidates for flat display devices instead of liquid crystal displays. The organic light emitting diode displays differ from the liquid crystal displays in that organic light emitting diode elements generating light are employed. In other words, the organic light emitting diode displays do not require backlight which the liquid crystal displays need. An organic light emitting diode element has high speed response, high contrast, and high visibility. Further, an organic light emitting diode display using the organic light emitting diode element has a relatively simple structure, which is advantageous in view of manufacturing cost.
FIG. 8 is a sectional view of a display cell of a conventional organic light emitting diode display. A thin film transistor 102 serving as a switching element, a thin film transistor 103 serving as a driver element, and a conductive layer 110 are formed on a substrate 101. A planarizing layer 104 is formed to cover the substrate 101, and the thin film transistors 102 and 103. An organic light emitting diode element 105, and conductive layers 106 and 107 are formed on the planarizing layer 104 so that the organic light emitting diode element 105 is located between the conductive layers 106 and 107. The conductive layer 107 is electrically connected to the thin film transistor 103 via conductive layers 108 and 110. The organic light emitting diode element 105 is electrically connected to and controlled by the thin film transistors 102 and 103. A protective layer 109 is deposited on the uppermost layer as shown FIG. 8. Such a protective layer is disclosed in, for example, “A 13.0-inch AM-OLED display with top emitting structure and adaptive current mode programmed pixel circuit”, T. Sasaoka et al, SID Tech. Dig., 2001, pp. 384 to 387.
The organic light emitting diode element 105 has a structure similar to that of a light emitting diode, including at least one of a hole transport layer and an electron transport layer, and an emitting layer. These hole transport layer, the electron transport layer, and the emitting layer are made of organic materials such as diamine compounds, quinolinol aluminum complex, and phthalocyanine. Some carbon-carbon conjugated bonds of these materials are easily separated by moisture or oxygen. Separating of the carbon-carbon conjugated bonds causes electrical conductivity to decrease. Therefore, the organic light emitting diode display has a sealing structure for protecting the surface of the organic light emitting diode elements from air.
To date, for such a sealing structure, the organic light emitting diode display is protected from air with a glass substrate separated from the surface of the organic light emitting diode elements by spacers. However, the glass substrate causes various problems such as occurrence of optical loss. For example, light emitted from the organic light emitting diode element is reflected by the glass surface, and weight and thickness of the organic light emitting diode display increases in addition to an increase in the cost of the glass substrate. Therefore, using the protective layer 109 made of silicon nitride (hereinafter, “SiNx”) having excellent light transmission characteristics, instead of the glass substrate, is desired.
However, the protective layer has a problem in which the protective layer located on the surface of the organic light emitting diode display causes a tensile stress. The tensile stress causes separation of the protective layer or substrate crack.
The material of the organic light emitting diode element is weak against a high temperature, and the glass transition occurs at a temperature as low as about 120 degrees centigrade. Therefore, depositing the protective layer requires the temperature condition not higher than the glass-transition temperature, more specifically, from about 80 to about 120 degrees centigrade.
Generally, it is common to deposit SiNx at a temperature of from 250 to 300 degrees centigrade, and preferably about 280 degrees centigrade. The SiNx layer deposited under a temperature of from about 80 to about 120 degrees centigrade has a lower density. The lower density leads to strong interatomic force. As a result, an influence of stress to the organic light emitting diode elements increases as compared with the normal layer structure.
Further, in order to protect the organic light emitting diode elements from air sufficiently, the protective layer needs to have a certain thickness, specifically, a thickness of from 1 to 3 micrometers. Generally, since the stress increases with an increase in the layer thickness, the tensile stress on the surface of the organic light emitting diode elements becomes a serious problem.
The protective layer, not limited to the SiNx layer, potentially has the problem of the stress. Therefore, in the general organic light emitting diode display which has the sealing structure with the protective layer, it is desired to decrease the stress caused by the protective layer.